How this is done is explained by the current theories of perception. External objects usually affect various senses at once, the impressions of which are thereby associated. Repeated experiences of one object are also associated on account of their similarity; hence a double tendency to merge and unify into a single percept, to which a name is attached, the group of those memories and reactions which in fact had one external thing for their cause. But this percept, once formed, is clearly different from those particular experiences out of which it grew. It is permanent, they are variable. They are but partial views and glimpses of it. The constituted notion therefore comes to be the reality, and the materials of it merely the appearance. The distinction between substance and quality, reality and appearance, matter and mind, has no other origin. The objects thus conceived and distinguished from our ideas of them, are at first compacted of all the impressions, feelings, and memories, which offer themselves for association and fall within the vortex of the amalgamating imagination. Every sensation we get from a thing is originally treated as one of its qualities. Experiment, however, and the practical need of a simpler conception of the structure of objects lead us gradually to reduce the qualities of the object to a minimum, and to regard most perceptions as an effect of those few qualities upon us. These few primary qualities, like extension which we persist in treating as independently real and as the quality of a substance, are those which suffice to explain the order of our experiences. All the rest, like colour, are relegated to the subjective sphere, as merely effects upon our minds, and apparent or secondary qualities of the object. But this distinction has only a practical justification. Convenience and economy of thought alone determine what combination of our sensations we shall continue to objectify and treat as the cause of the rest. The right and tendency to be objective is equal in all, since they are all prior to the artifice of thought by which we separate the concept from its materials, the thing from our experiences. The qualities which we now conceive to belong to real objects are for the moat part images of sight and touch. One of the first classes of effects to be treated as secondary were naturally pleasures and pains, since it could commonly conduce very little to intelligent and successful action to conceive our pleasures and pains as resident in objects. But emotions are essentially capable of objectification, as well as impressions of sense; and one may well believe that a primitive and inexperienced consciousness would rather people the world with ghosts of its own terrors and passions than with projections of those luminous and mathematical concepts which as yet it could hardly have formed.